1. Technical Field
The present invention relates to a vision shift amount measuring method and a vision shift amount measuring jig used for spectacle lenses.
2. Related Art
When viewing an object, a person rotates his/her head or eyes for obtaining side view and upper and lower view (hereinafter referred to as viewing action), The rotation angles of the head and eyes during the viewing action are different for each person, and thus various types of spectacle lens designing method and adjusting method capable of dealing with the viewing action of each person have been proposed (for example, see JP-T-2003-523244 and JP-T-2008-521027).
For designing spectacle lenses suitable for each person, measuring a vision shift amount of the person is essential. The vision shift amount herein refers to a distance of vision shift of a wearer of spectacles on a lens, defined as a concept which includes an eyeball downward movement amount corresponding to a distance of vision shift on the lens when the wearer of the spectacles shifts vision in the up-down direction to change the viewing condition from horizontal view to near view (to the condition of reading a book, for example), and a side view amount corresponding to a distance of vision shift on the lens when the wearer of the spectacles shifts vision in the left-right direction to change the viewing condition from horizontal view to side view.
In designing a progressive-power lens, the eyeball downward movement amount is particularly important. The progressive-power lens is an aspherical lens which includes a distance portion having a power (dioptric power) corresponding to distant view, and a near portion having power corresponding to near view. The distance portion is located in an upper area of the lens, and the near portion is located in a lower area of the lens. A progressive band whose power progressively changes is further provided between the distance portion and the near portion. These portions and band have no boundary between one another, and a wearer of the progressive-power lens can view objects located in a distant place and in a near place through a single lens. The distance portion, the near portion, and the progressive band are required to be adjusted according to the purpose of use for each person (for distant-near view use, middle-near view use, near-near view use, full-time use, part-time use, still use, dynamic use and other purposes) (optical fitting). The eyeball downward movement amount is measured as a distance between a distance eye point as the vision position of the wearer of the spectacles on the lens in the horizontal view condition and a near eye point as the vision position of the wearer on the lens in the near view condition.
Various methods for detecting the distance eye point and the near eye point have been proposed (for example, see JP-A-2006-91411). According to JP-A-2006-91411, a group of light-transmissive color bars having two or more different colors disposed adjacent to each other are provided on each of detection areas for the distance eye point and the near eye point of a transparent plate body attachable to a wearing frame or a spectacle lens. For detecting the eye points, the wearer views direct light or reflection light emitted from a pen-type light or the like, and recognizes the position of the light passing through the detection areas.
According to JP-T-2003-523244 and JP-T-2008-521027, quantitative values are obtained by using a head and eyeball movement measuring device such that a lens optimized for each person can be produced based on the obtained values. However, in determining the near eye point on the progressive-power lens, the posture of the wearer is a factor largely concerned with the determination as well as the head and eyeball movements, Thus, it is difficult in some cases to determine the near eye point with high accuracy by the method of these references.
Moreover, the measurement of the eyeball movement disclosed in JP-T-2003-523244 and JP-T-2008-521027 requires a long time of detection which imposes a too much detection burden on a customer, and also requires a highly expensive detection device to be prepared.
On the other hand, when the group of light-transmissive color bars are viewed at a distance of 12 mm before the eyes in the method disclosed in JP-A-2006-91411, modulated colors are recognized (mixed colors or lighter colors) due to the effect of out-of-focus condition or diffraction. In this case, it is difficult in some cases to securely determine the color differences (lowering of visual recognizability).
In addition, the operation for detecting both the distance eye point and the near eye point and calculating the difference between the relative positions of these points is a complicated process, which lowers accuracy in some cases and prevents practical use of the method.